1. Field of the Invention
This invention relates to long range wide angle zoom lenses and more particularly to compact and high performance long range wide angle zoom lenses suited for use in TTL type 35 mm cameras.
2. Description of the Prior Art
It has already been known to provide zoom lenses of the type having a divergent 1st lens group counting from the front, a convergent 2nd lens group and a divergent 3rd lens group with the axial separations between the successive two of these three lens groups being made variable to effect zooming as proposed, for example, in B. Pat. No. 398,307, U.S. Pat. No. 4,196,969 and Japanese Laid Open Pat. No. Sho 54-26754. Of these conventional examples, however, those which have a long range of magnification change call for a large increase of the bulk and size of the lens system, and those which have little distortion with the zoom ratio on the order of about 2 cannot be said to give a high varifocal range. Also, the separation between the 2nd and 3rd lens groups is wider when in the wide angle position than when in the telephoto position. In other words, the total movement of the 3rd lens group for zooming is made larger than that of the 2nd lens group. In general, with the aforesaid power distribution over the lens groups, when it is intended to realize a greatly increased varifocal range while still preserving the bulk and size of the lens system to a minimum, the backfocal distance in the wide angle position tends to become short. Also, the maximum possible diameter of the diaphragm aperture in the telephoto position tends to increase when the diaphragm is located within the 2nd lens group.
In the zoom lens having the power distribution as in the prior art, therefore, if the manner in which the total movements of the 2nd and 3rd lens groups are differentiated remains unchanged from that described above, a problem arises wherein, as the zoom ratio increases, the back-focal distance in the wide angle position becomes shorter, and the aperture diameter in the telephoto position goes in a direction to increase.
In the interchangeable objectives for the 35 mm single lens reflex cameras, there is need of dimensioning the back-focal distance so as to allow for at least a space in which the mirror can operate. Also, since the increase in the aperture diameter calls for an increase in the outer diameter of the lens mounting, it is desirable that the aperture diameter be as reduced as possible. According to the prior art, however, it was very difficult to realize a zoom objective having an increased varifocal range with a minimization of the bulk and size of the lens system.